The present invention relates to a control system for an internal combustion engine for performing catalyst temperature rise acceleration control for accelerating a rise in temperature of a catalyst for exhaust gas purification at cold starting of the engine, and more particularly to such a control system for an internal combustion engine mounted on a vehicle having a brake booster to which a negative pressure (a pressure that is lower than the atmospheric pressure) in an intake system of the engine is introduced.
A catalyst for exhaust gas purification is provided in the exhaust system of an internal combustion engine. This catalyst is inactive at low temperatures. Consequently, the catalyst does not purify the exhaust gas at low temperatures. Therefore, it is desirable to rapidly raise the temperature of the catalyst, immediately after starting the engine, to activate the catalyst.
Known in the art is a technique for accelerating the rise in temperature of the catalyst. This technique increases the intake air amount immediately after starting of the engine as compared with the air intake amount at normal idling of the engine. The technique also controls the ignition timing in a retarding direction so that the rotational speed of the engine coincides with a target rotational speed (Japanese Patent Laid-open No. 10-299631). According to this technique, the fuel supply amount increases with the increase in the intake air amount. Thus, the combustion heat increases immediately after starting of the engine as compared with that at normal idling. The immediate combustion heat increase enables the acceleration of the temperature rise in the catalyst.
A brake booster is widely used for assisting the braking force for the vehicle to reduce a depression force applied to the brake pedal. A negative pressure generated in the intake pipe at a position downstream of the throttle valve of an internal combustion engine is introduced to the brake booster. The brake booster has a diaphragm to which a negative pressure corresponding to the depression amount of the brake pedal is applied to increase the braking force. Accordingly, when the negative pressure in the brake booster decreases (the absolute pressure in the brake booster increases), the assistance of the braking force by the brake booster becomes insufficient.
When performing the above-mentioned catalyst temperature rise acceleration control, the intake pressure in the intake pipe increases with an increase in the intake air amount. Accordingly, there is a possibility that the negative pressure in the intake pipe cannot be introduced to the brake booster even when the negative pressure in the brake booster is small (the absolute pressure in the brake booster is high).
It is accordingly an object of the present invention to provide a control system for an internal combustion engine which can sufficiently ensure a braking force by the brake booster when performing the catalyst temperature rise acceleration control.
The present invention provides a control system for an internal combustion engine having an intake system and an exhaust system. The exhaust system includes a catalyst, and the intake system includes a throttle valve. The engine is mounted on a vehicle having a brake booster to which an intake pressure at downstream of the throttle valve is introduced. The control system includes intake air amount control means, ignition timing control means, catalyst temperature raising means, intake pressure detecting means, and catalyst temperature rise control suppressing means. The intake air amount control means controls an intake air amount of the engine. The ignition timing control means controls an ignition timing of the engine. The catalyst temperature raising means increases the intake air amount after starting of the engine and retards the ignition timing according to a rotational speed of the engine. The intake pressure detecting means detects the intake pressure at downstream of the throttle valve. The catalyst temperature rise control suppressing means gradually suppresses the operation of the catalyst temperature raising means when a pressure difference between the detected intake pressure and the atmospheric pressure during the operation of the catalyst temperature raising means is less than a predetermined pressure.
With this configuration, when the pressure difference between the intake pressure at downstream of the throttle valve and the atmospheric pressure is less than the predetermined pressure during the operation of the catalyst temperature raising means, the operation of the catalyst temperature raising means is gradually suppressed, so that the intake pressure decreases. Accordingly, when a pressure in the brake booster is high, the pressure in the brake booster can be made to decrease (the negative pressure in the brake booster can be made to increase), thereby sufficiently ensuring a braking force by the brake booster.
Preferably, the catalyst temperature rise control suppressing means gradually decreases the increased intake air amount and gradually decreases the retard amount of the ignition timing.
Preferably, the catalyst temperature raising means operates when the rotational speed of the engine is greater than or equal to a predetermined lower limit and the engine is in an idling condition.
Preferably, the control system further includes coolant temperature detecting means for detecting a coolant temperature of the engine. An operating time period of the catalyst temperature raising means is set according to the coolant temperature detected at starting of the engine.
Preferably, the catalyst temperature raising means sets the retard amount of the ignition timing so that the rotational speed of the engine becomes equal to a target rotational speed.
The present invention provides another control system that includes the intake air amount control means, the ignition timing control means, the catalyst temperature raising means, and the catalyst temperature rise control suppressing means. This control system further includes brake booster pressure detecting means instead of the intake pressure detecting means. According to this control system, the catalyst temperature rise control suppressing means gradually suppresses the operation of the catalyst temperature raising means when a pressure difference between the detected brake booster pressure and the atmospheric pressure during the operation of the catalyst temperature raising means is less than a predetermined pressure.
With this configuration, when the pressure difference between the brake booster pressure and the atmospheric pressure is less than the predetermined pressure during the operation of the catalyst temperature raising means, the operation of the catalyst temperature raising means is gradually suppressed, so that the intake pressure decreases. Accordingly, the brake booster pressure can be made to decrease, thereby sufficiently ensuring a braking force by the brake booster.